Recent advancements in the ability to integrate microelectronic chips has led to a paradigmatic change in circuit system design. Through Very Large Scale Integration (VLSI), and Ultra Large Scale Integration (ULSI), it is now possible to fabricate tens and hundreds of millions of chips onto a single semiconductor wafer. These advancements in integration technology and manufacturing have facilitated the creation of Systems-On-Chips (SOC). Previously, the creation of an electronic system required the integration of several microchips and discrete electronic components on to a Printed Circuit Board (PCB). However, with an SOC, all of these separate microchips and discrete electronic components are integrated onto a single microchip. An SOC will contain a large range of electronic devices including Central Processing Units (CPUs), random logic, memory, digital, and analog circuitry.
SOCs have significant advantages over electronic systems created on boards with discrete components. One advantage is size. An integrated circuit having an SOC is much smaller than a circuit board based system. Both transistors and interconnect wires are reduced to micrometer sizes, in contrast to the millimeter or centimeter scales of discrete components. Through using an SOC, it is possible to improve the overall size and portability of electronic devices.
The reduction in size afforded by SOCs also leads to improvements in power consumption and device speed. Circuit operations that occur on a single integrated circuit require much less power than a similar circuit implemented on a PCB with discrete components. Through using less power, SOCs are highly desirable for portable electronic devices. With respect to device speed, signals are transmitted at far greater speeds within a single chip than within a PCB. The speed of signals within a single microchip is hundreds of times faster than on a PCB. Both the power consumption and speed of an SOC is owed to the smaller parasitic capacitances and resistances of devices fabricated on a single IC.
Replacing electronic circuits supported on a PCB with an SOC greatly reduces cost. Reducing the number of discrete electronic components, lowering power supply requirements, and placing electronic system components onto a single IC leads to cost reductions in the electronic system.
Cost effective SOC design requires the use and integration of pre-designed blocks. Through creating pre-designed general purpose circuit blocks, commonly referred to as Intellectual Property (IP) blocks, it is possible to reduce the time to design an SOC. These IP blocks can include digital blocks, analog blocks, RF blocks, CPU blocks, memory blocks, and random logic blocks.
Today, mixed-signal circuits constitute an important part of integrated circuits. Many applications require both analog front end systems and digital processing blocks. The integration, however, of these various digital and analog circuit blocks into an SOC presents several unique problems. For example, placing each of these various circuit block types on a single IC often allows inter-circuit interaction through the IC substrate. Such interaction can greatly degrade and inhibit the expected operation of the SOC when digital and analog elements are placed on the same substrate.
The differential noise sensitivity of dissimilar circuit types spawns another problem. Analog circuitry is sensitive to electrical noise produced by other circuits and devices. This sensitivity to noise makes it desirable to isolate analog circuitry from sources of electrical noise. On the other hand, digital circuits are far less sensitive to electrical noise due to their digital nature. The low voltage swing of an analog device produces little noise. Further, the current bases for analog circuitry keeps noise levels low. Consequently, analog circuits produce low noise levels. However, digital circuits produce a significant amount of electrical noise due to the large rail to rail voltage swings of the devices. Integrating analog and digital circuit elements onto a single IC typically exposes the analog circuit elements to the high noise component produced by the digital circuit elements. To integrate analog and digital circuit components on a single IC, analog circuit components must be isolated and insulated from the electrical noise produced by digital circuit components.
Signal crosstalk also plagues dissimilar device circuitry. Crosstalk is interference caused by two or more signals becoming partially superimposed on each other due to electromagnetic (inductive) or electrostatic (capacitive) coupling between devices or conductors carrying the signals. In MOS circuits, this interference between devices can produce false switching in other parts of the system. Consequently, it is highly desirable to develop an IC that can support analog and digital components while reducing crosstalk and noise to ensure high performance and reliability.